LLC topology (as shown in FIG. 1) is widely used in various kinds of products, such as LCD TV, networking power supply etc. LLC has at least following advantages: a) achieving ZVS for switching transistors without extra circuits, meeting the requirement of high frequency and high efficiency at the same time; b) ZCS for secondary rectification at the secondary side and low voltage stress, which contribute to high efficiency, therefore being very suitable for two stage topology with PFC as its first stage to improve the efficiency of AC input and DC output; c) no output inductance and low cost; and d) easy for magnetic integration in the applications lower than 500 W.
LLC topology is widely used due to the above advantages. However, LLC has no output filter inductance, which means, in order to solve the problem of large current ripple, a lot of capacitors connected in parallel in the output terminal are required to absorb the current ripple, especially in the low output voltage/high current/high power applications. Compared with the regular LLC topology, interleaved LLC topology (as shown in FIG. 2) can solve the current ripple problem significantly, and reduce the stress on the capacitors, which makes interleaved LLC a better solution in high power or high current level applications. In the interleaved LLC, two LLC convertors operate in parallel under the condition that there is 90° phase difference between the drivers. The parameters of the two resonant cavities may not be completely consistent with each other in practical application. Given the interleaved LLC has the same output voltage, the gain would be the same. When there is a tolerance between the parameters of the two resonant cavities, the following would happen:fr1≠fr2→fn1≠fn2 G1=G2→Q1≠Q2→I01≠I02.
As shown above, when the parameters of the two resonant cavities are not consistent with each other, the two transformers, T1& T2, of the interleaved LLC, will be under different load conditions. And the worse the tolerance between the resonant parameters is, the more serious the current unbalance would be. As shown in FIG. 3, when a 900 W HB (Half-Bridge) interleaved LLC having resonant parameters with ±5% tolerance operates at full load, the current output I01 of one of the LLC convertors would be almost zero and thus under light load condition. In practical application, under the worst tolerance condition, one of the LLC is at light load, while the other LLC convertor has been already overloaded or even damaged when the condition is really serious. Unfortunately, to accurately learn the resonant parameters (resonant inductance Lr, resonant capacitance Cr) is always very difficult for an interleaved LLC, which becomes the bottleneck of the application of the interleaved LLC.